Magic Jack.....How many GB does a phone call use?

I think they meant kilobits per second, kbps. They gave you enough
information to estimate the answer.

If they sample your voice at 8khz, (the bandwidth is 4khz and per
Nyquist you sample at twice the rate), and use 16 bits per sample,
that would be 8000x16 = 128000 or 128kilobits per second. Wow, what
an expensive college education from decades ago can do for you. I
think a lot of voice is lost over 3500Hz, so that may explain why the
range goes lower. Of course this does not account for parity bits or
error correction, but 80-120kbps is in the ballpark.

Now all you have to do is convert for a 10 minute call (note kilo is
used base 10 for data rates, but data storage uses base 2, so kilo is
1024):

10*60 = 600 seconds. So, you multiply 120kbps by seconds to get
kilobits (remember learning units in chemistry or physics?) 600*120 =
72000 kilobits. There are 8 bits in a byte, so divide by 8 to get
9000 kilobytes. There are 1024 kilobytes in a megabyte, so divide by
1024. 9000/1024 = 8.79MB, which is greater than 5MB.

It looks like Magic Jack uses too much bandwidth.

I am frankly extremely surprised by this result. Voice-only exceeds
the data limit of a high speed internet connection over a 10 minute
period? Maybe my seat of the pants calculation is off, but it gives
you a starting point.

I never liked their commercials anyway. Now I'll be saying "you need
Magic Jack" to myself all day...

Off the top of my head, I can't say for sure. However, if you're
using EVDO with Verizon Wireless, I'd strongly recommend against using
something like MagicJack (or any VOIP). Besides Verizon looking down
on this sort of thing (they could cancel you for doing it), there are
reasonably steep data requirements.

Also, EVDO does not have the consistent throughput needed for quality
VOIP.

However, since you have 30 days, give it a try and see if you're happy
with the quality. There are ways to measure your data usage, but they
can vary widely from what Verizon says. You might to call them and
ask if they can provide you accurate stats, perhaps for a 1 or 2 day
period.

By the way 120kbs would be about 15 kbytes/sec. Which would mean that
an hour call would be about 50MB. If that's the case, and you weren't
otherwise using data (which I'm assuming is not the case), you're
looking at about 20 hours a month. That might be fine for your needs.
(My math might be very off here, so someone else might need to
correct me).

For reference, an average song download (3-5 minutes) is about 5-10MB.

one of those "test my connection speed" sites?

it might not be a bad idea